Technical Field
Embodiments of the subject matter disclosed herein generally relate to processing seismic data acquired using a streamer towed to have a variable-depth profile, more specifically, to identifying multiples included in the up-going wavefield using up-going and down-going wavefields at a predetermined datum.
Discussion of the Background
In seismic acquisition, energy (i.e., a seismic wave) generated by a seismic source propagates downward into a geological formation, and part of the energy is reflected back up. Characteristics of the reflected energy detected by seismic sensors are used to produce an image of the earth's reflectivity. In marine data acquisition, the sensors are housed by a streamer towed underwater.
Two types of multiple reflections complicate seismic data processing: water surface-related multiples and inter-bed multiples. Surface-related multiples occur when energy reflected from the subsurface reaches the air-water surface and is reflected back downward into the water column and the geological formation under the seabed. This energy reflected at the air-water surface produces a second train of energy reflected from the geological formation. Inter-bed multiples are similar, but in this case the downward-reflecting surface is a rock interface inside the geological formation.
Over the years, many methods have been developed to suppress multiples. One method, known as “radon demultiple,” is a modeling approach usually in the common midpoint (CMP) domain, where the difference in moveout is used to discriminate between primary energy, which is usually faster, and multiple energy, which arrives later in time. The data is first normal-moveout corrected and then a parabolic radon model is generated. In the radon model, primary and multiple energy are distributed on different parabolas which allows for the multiple energy to be identified and consequently selected. The multiple energy is then reverse-transformed back to the offset-time domain. Finally, the multiple energy is subtracted from the original input data. The “radon demultiple” method is only effective when moveout discrimination between primary and multiples energy is apparent. Therefore, this method is often ineffective in shallow water environments or at near offsets.
Another method known as “surface-related multiple elimination” (SRME) uses convolutions and summations to estimate the multiple energy. With sufficient sampling of sources and receivers, this approach may produce a multiple model with the correct kinematic timing. Any amplitude, timing or phase errors are usually subsequently corrected with adaptive subtraction of the multiple model from the original data. However, the SRME method is difficult to use when the input data is not sufficiently well spatially sampled. In addition, the SRME method relies on an adaptive subtraction step.
Yet another method known as “deconvolution” may be applied either in the offset-time or a model (e.g. tau-p) domain. This method builds a prediction operator based on an auto-correlation of the data. The auto-correlation, which contains energy at the multiples periods, is used to derive a deconvolution operator (based on user parameters) to be applied to the data. This deconvolution method is often only suitable for shallow water and for simple structures.
Another method known as “wave-equation modeling” produces a multiple model by forward extrapolating seismic reflections into the subsurface and back to the receiver datum. The extrapolation step requires a reflectivity and velocity model. This method is suitable for modeling long-period multiples and relies on an adaptive subtraction step as well as some knowledge of the subsurface (e.g., reflectivity and velocity).
According to yet another method known as “inverse scattering,” a multiple model is predicted by constructing a subseries of the scattering series, which corresponds to the multiples and may be used for 2D and 3D multiple prediction. This method is described in the article, “An inverse-scattering series method for attenuating multiples in seismic reflection data,” published in 1997 in Geophysics, 62, No. 6, pages 1975-1989, the content of which is incorporated in its entirety herein by reference. This method also requires dense source and receiver (i.e., detector) sampling.
As pointed out in the book, “Seismic multiple removal techniques past, present and future,” by Verschuur, D. J., a 2006 EAGE publication, the content of which is incorporated in its entirety herein by reference, each of the methods discussed in this section can be ineffective under certain conditions/environments.
Therefore, multiples removal remains a subject of continuing research, with new opportunities and challenges occurring as data acquisition systems evolve, for example, by towing streamers according to a variable depth profile instead of towing at a substantially constant depth.